1. Field of the Invention
Devices, systems, and methods for detecting the presence or absence of an object, e.g., a cap on a sample vessel or the sample vessel itself, are disclosed and, more specifically, devices, systems, and methods that include passive, auxiliary reflective areas that are adapted to illuminate an edge portion of the object passively for better contrast against a dark background.
2. Description of the Related Art
Reliable, rapid detection of the presence or absence of an object, such as a cap on a sample vessel or the sample vessel itself, is desirable in laboratory automation and other applications. One method of detecting objects is through image acquisition by which an image of a target is collected and compared to a reference image.
For image acquisition to be reliable, the images must have acceptable definition, which is to say that there must be sufficient contrast between the object being imaged and the background, to determine whether or not the object is present, i.e., that the cap is on or is not on the sample vessel.
Illumination of the object can be active or passive or a combination of the two. Intuitively, use of a light, e.g., white, background for passive illumination in combination with dark objects should be preferred. However, in practice, light backgrounds are subject to shading. Accordingly, darker backgrounds, which generally have relatively lower reflectivity than light backgrounds, are preferred.
However, achieving an acceptable definition, for example, using a dark background and relatively dark objects is more difficult because there may be an indistinguishable contrast between the object and the dark background. Even using a high quality, optically black background for contrast may not provide reliable results.
Referring to FIG. 1, a common object detection system in accordance with the prior art is shown. The system 10 includes a dark, e.g., black, background 12 and an imaging device, e.g., a camera 14. The camera 14 includes a lens 16 and a pair of illumination devices or light sources 11 and 13 that are adapted to generate active illumination. Optionally, the illumination devices 11 and 13 do not have to be part of the camera 14 (as shown), but can, instead, be separate elements that are disposable at, near or remote from the camera 14.
Illumination device 11 is adapted to generate a beam of light 17 and illumination device 13 is adapted to generate a beam of light 18, which collectively actively illuminate the object 15 and the dark background 12. Light reflected from the dark background (if any) and the object 15 that is within the field of view 19 of the lens 16 are collected by the imaging device 14, which is adapted to produce an image of the object 15.
As is well-known, incident light that is reflected includes diffuse reflections and specular reflections. Specular reflection is near “perfect” reflection. Light from a single direction is reflected in a single direction, such that the angle of incidence (θi) is equal to the angle of reflection (θr). In contrast, diffuse reflections result in incident light being reflected or scattered in multiple directions.
Diffuse reflection body angle distribution depends, in large part, on the quality of the surface, but can generally be approximated using the following equation:R•((1−C)cos(θi−θd)+C)in which R corresponds to the brightness of the material, e.g., R=1−A, where A is a light absorption coefficient; θd corresponds to the angle at which light is registered, e.g., by a lens; and C is a scatter coefficient indicative of how much non-absorbed light is reflected in directions other than those directions corresponding to specular reflection. For most materials, however, even materials that are dark and/or matte, more light is reflected in the direction of θr as shown in FIG. 2, especially when the angles of incidence and reflection decrease with respect to the object plane normal.
Problematically, as shown with FIG. 1, light 17 and 18 incident on an insufficiently dark background 12 that is reflected and registered by the lens 16 of the camera 14 often causes the background 12 to appear brighter relative to the object 15. Moreover, referring to FIG. 3, at contrast points, i.e., edge portions 46 and 48, specular reflections 49 of light 47 incident on an object 15 and specular reflections 44 of light 43 incident on the dark background 12 that pass near the edges 46 and 48 of the object 15, but without being incident thereon, will cause the image of the object 15 to appear to be much darker relative to the dark background 12, making recognition of an object or object feature (e.g., a vessel or a vessel cap) more difficult.